a) Field of the Invention
The invention is directed to an arrangement for the suppression of particle emission in the generation of radiation based on hot plasma in x-ray radiation sources, particularly EUV radiation sources.
b) Description of the Related Art
Soft x-ray radiation and extreme ultraviolet (EUV) radiation are applied in many areas of technology. Due to the development of highly reflecting multilayer dielectric mirrors which are suitable as collector optics for this spectral range, EUV radiation is becoming increasingly important in semiconductor lithography for the exposure of increasingly smaller structures of ULSI (Ultra Large Scale Integration) circuits.
The requirements for radiation sources for the above-mentioned jobs in semiconductor lithography could possibly also be met by a synchrotron, but the latter is very expensive and is also very inflexible due to its size. Therefore, synchrotrons have not found acceptance in the semiconductor industry for exposure of ULSI circuits.
The EUV radiation sources that have now moved into the spotlight can only be successfully used in the future if they emit sufficiently intensive radiation in the wavelength range of 13.5 nm and, at the same time, less debris. The generation of debris, that is, the emission of neutral and charged particles from the radiation-emitting plasma, is considered at present to be the most pressing problem because the life of the collector optics is considerably reduced thereby. The life of collector optics in lithography machines should be at least one year. This can not be achieved with a high debris emission even when the radiation source has a sufficiently high output to compensate to some extent for the decreasing reflectivity of the multilayer mirrors.
Various types of debris filters are currently in use but, with their characteristics, do not achieve reduction of debris to the required degree. For example, existing filters are formed of honeycombs or concentric cones. However, the baffles reduce the transmission of radiation geometrically due to shadows cast by the edges and diffraction phenomena. Increasing the quantity of filter walls or decreasing the distances between the walls in order to increase the filtering effect necessarily limits the beam bundle more narrowly.
On the other hand, an arrangement of plates lateral to the propagation direction of the radiation can guarantee a high transmission. A filter arrangement of this type for an x-ray source is known from U.S. Pat. No. 4,837,794. This reference discloses the arrangement of conical baffles outside of the beam bundle defined by the exit window and the use of a constantly renewable UV filter and a magnetic field for deflecting primary electrons in the vacuum chamber before the exit window of the radiation source in order to prevent hot gases from escaping and to prevent other unwanted components of plasma-generated x-rays. However, the usable solid angle is greatly reduced by this arrangement. When the plates are spaced farther apart in order to enlarge the solid angle, the filtering effect is reduced. Fast charged and uncharged particles are filtered out of the exiting beam bundle only to an inadequate extent with this arrangement.
Further, known mechanical shutters which quickly close the beam path after every beam pulse in order to block the slower particles can not achieve the required repetition rates (up to 10 kHz) for technical reasons.